1. Field of the Invention
The invention relates to liquid and solid bromine-containing compositions, and a liquid mixed halogen bromine and chlorine-containing composition, for use as biocides in water treatment. The highly concentrated liquid compositions and high-activity solid compositions have excellent physical and chemical stability. The invention also relates to methods of preparing the liquid and solid compositions.
2. Description of the Related Art
There are many ways of delivering a biocidal dose of bromine into water systems where the growth of microorganisms must be controlled. Early examples of accomplishing this involved introducing elemental liquid bromine (Br2) or liquid bromine chloride (BrCl) to the water requiring treatment. However, both liquids are very volatile and evolve copious amounts of highly toxic and corrosive bromine fumes under normal conditions. Therefore, elaborate and expensive storage, transportation and handling safeguards must be practiced when using these products. It is hardly surprising that the high costs of managing these hazardous properties have rendered the use of elemental liquid bromine and liquid bromine chloride obsolete in most water disinfection applications.
An alternative system that overcomes these limitations employs an aqueous solution of sodium bromide (NaBr) in conjunction with liquid sodium hypochlorite (NaOCl) bleach. The user feeds the two materials to a common point where the NaOCl oxidizes the bromide ion to yield a mixture of hypobromous acid (HOBr) and hypobromite (OBr−) ion. This activated solution must then be quickly introduced to the water being treated because the species in solution are unstable to the following rapid disproportionation and decomposition reactions.
Disproportionation:2HOBr + OBr− = BrO3− + 2HBrDecomposition:2HOBr = O2 + 2HBrSuch reactions are undesirable because the products of the reactions (HBr, O2 and BrO3−) are not biocidally active.
Many users of this technology indicate that metering of two separate solutions of NaBr and NaOCl is especially inconvenient since two sets of pumps, flowmeters, valves, pipe work, and container dikes are necessary. In addition, the activation system must be designed so that sufficient time is allowed for the NaBr and NaOCl solutions to fully react prior to being injected into the water being treated. Another disadvantage is that NaOCl solutions deteriorate rapidly, and so delivery pumps must continually be readjusted to compensate and ensure that the correct proportions of reactants are maintained for accurate dosing. This dual NaBr/NaOCl activation system is so cumbersome and difficult to control that many users have demanded a system in which the NaBr is supplied in a pre-activated, one-drum form that is stabilized to disproportionation and decomposition reactions. Clearly, a single feed liquid bromine biocide with these properties would be far more convenient and easier to use and control than the dual component approach.
One of the earlier attempts to develop a single feed, liquid bromine biocide is described in U.S. Pat. Nos. 4,886,915 and 4,966,716 to Favstritsky, et al. An aqueous solution containing 38% elemental bromine complexed as ethanolammonium hydrogen perbromide was administered to the water being treated. Subsequently, U.S. Pat. No. 5,141,652 taught the use of strong solutions of halide salts and hydrohalic acids to form solutions of bromine chloride complexes. However, neither technology became fully commercialized because the problem of bromine fuming from the products was not completely alleviated.
U.S. Pat. Nos. 5,683,654, 5,795,487, 5,942,126, and 6,136,205 describe processes to manufacture a single feed, liquid bromine biocide by mixing aqueous hypochlorite solutions with bromide ion sources and then stabilizing this reaction mixture to disproportionation and decomposition by introduction of a stabilizing agent. Although the resultant solutions did not evolve bromine fumes, the process required a complex two-step reaction. In the first step, NaBr and NaOCl solutions were mixed and sufficient time was allowed to permit the formation of a NaOBr solution. In the second step, this was then introduced to a solution of the stabilizing agent maintained at 50° C. However, the main limitation of this approach lay in the use of a NaOCl bleach solution as the foundation of the process. When NaOCl bleach was used as the activating agent, the concentration of the stabilized bromine product became limited by the concentration of NaOCl bleach that is commercially available. In fact, despite using the highest strength grades of industrial NaOCl bleach, the bromine content of the resulting stabilized solutions was only about 14% as Br2 (6.3% as Cl2).
Moore et al. overcame the complexity of the two-step reaction in U.S. Pat. Nos. 6,068,861, 6,495,169, and 6,322,822 and disclosed a single vessel reaction in which bromine or bromine chloride was added to a halogen stabilizer solution under conditions of pH control. The available bromine content of the resultant solutions was reported to be at least 10% as Br2 (4.4% as Cl2). However, when the process was scaled up, as disclosed in U.S. Pat. Nos. 6,299,909,6,306,441, and 6,348,219, it was found necessary to employ a three-reactor sequence, and an upper limit of 18% as Br2 (8% as Cl2) was imposed as the highest strength concentrate that could be made by the process. Another drawback to the processes described by Moore et al. was the hazardous nature of the starting reagents involved. The high cost of transporting and handling of elemental bromine or bromine chloride meant that commercial manufacturing of the stabilized bromine solutions could only be accomplished in highly specialized, dedicated plants close to sites where bromine is recovered from naturally occurring brines. In the U.S., the process of Moore et al. could only be practiced economically in manufacturing plants close to the brine fields of southern Arkansas or Michigan.
Yang et al. sought to increase the concentration of the available bromine in the stabilized formulation, and disclosed processes in U.S. Pat. Nos. 6,156,229, 6,423,267, and 6,287,473 that also employed the use of elemental bromine, bromine chloride, or sodium bromate. Using these methods, liquid bromine concentrates containing 26.2% as Br2 (11.6% as Cl2) were prepared. However, it was reported that such solutions were physically unstable, and large amounts of bromine-containing solids precipitated out of solution on standing overnight. Upon filtration of the solids, the liquid phase comprised 18.7% as Br2 (8.3% as Cl2). No information on the long-term chemical stability of the concentrates was reported. Other efforts to prepare stabilized liquid bromine formulations of high concentration were disclosed in U.S. Pat. No. 6,270,722 where high strength industrial grade sodium hypochlorite solutions and gaseous Cl2 were used to oxidize sodium bromide solutions. Stabilized liquid bromine concentrates containing up to 20.75% as Br2 (9.22% as Cl2) were reported, but no information regarding the stability of the products to precipitation of solids was disclosed.
As is well noted in the prior art, the bromine content of stabilized formulations prepared using only sodium hypochlorite bleach solutions is limited by the strength of commercially available material. Indeed, even using the highest strength grade of industrial NaOCl bleach, the bromine content of the resulting solutions is reported to be only around 14% as Br2 (6.3% as Cl2). Although higher concentration solutions can be prepared with processes that employ Br2, BrCl, or Cl2, those processes result in a substantial amount of halide ion salt contaminants. Typically, when using these reagents, half of the respective Br atoms, and all of the respective chlorine atoms, materialize as halide ion salts. These halide ion salts are soluble and remain in the solution. These halide ion salts are deleterious because they: (1) limit the concentration of stabilized liquid bromine that can be attained in solution; (2) decrease the chemical stability of the resulting liquid bromine concentrates; and (3) decrease the physical stability of the liquid bromine concentrate resulting in undesirable precipitation of salts from solution.
Certainly, the physical and chemical stability of the liquid bromine solutions was of concern to McKinnie, et al. in U.S. Pat. No. 6,506,418. They reported that under the acidic conditions advocated in their earlier processes based on BrCl, Br2, or Cl2, “a substantial portion of the sulfamate can be hydrolyzed rather rapidly to sulfate” (col. 3, line 67 to col. 4, line 1) and further that “loss of sulfamate due to hydrolysis to sulfate can result in decreased storage stability of the finished product” (col. 4, line 3 to col. 4, line 5), later to point out that “loss of sulfamate imposes an economic burden on the operation” (col. 4, line 8 to col. 4, line 9). To overcome these deficiencies, acidic reaction conditions were discouraged, and alkaline pH conditions of 8–10 were recommended when introducing BrCl, Br2, or Cl2 to the reaction medium. Yang et al. also recognized the benefits of alkaline conditions in U.S. Pat. Nos. 6,123,870 and 6,287,473 for processes to prepare liquid bromine solutions. In these processes, BrCl or Br2 was added to highly caustic solutions of sodium sulfamate such that the resulting solution had a pH of 12.5. It was claimed that without adequate pH control, rapid decomposition of the oxidizing species occurred.
Users of stabilized liquid bromine products recognize that formulations that contain higher levels of active ingredients have distinct economic advantages over dilute products since less product needs to be applied to a water system in order to achieve the equivalent dose of a weaker material. Also, more concentrated products need to be replaced less frequently than dilute products. Other advantages of concentrated liquids include reduced packaging, storage, and transportation costs per unit weight of active ingredient. Products prepared to contain high levels of dissolved active ingredients must display two essential attributes. First, they must be chemically stable, i.e. they should not decompose quickly, so as to maintain high activity for extended periods. Second, they must be physically stable, i.e. they should not precipitate or crystallize into solid salts that could plug pipe work and make the feeding of liquid materials grind to a halt.
Therefore, the need exists for liquid biocidal bromine compositions of higher concentration and superior physical and chemical stability than those described in the prior art and for methods of preparation of such compositions. There is a need for methods that employ inexpensive starting materials that are less hazardous and easier to handle than elemental liquid Br2 or liquid BrCl. There is also a need for methods that are not restricted to manufacturing plants close to bromine recovery facilities and that may be conducted quickly and efficiently in a single reactor. This invention addresses these needs.
Additionally, there is a need for liquid biocidal compositions based on mixtures of bromine and chlorine. No such compositions have ever been reported. All of the prior efforts have been directed toward the preparation of compositions in which bromine is the sole active ingredient, primarily because of the superior biocidal performance of bromine compounds compared to chlorine compounds. In waters exerting a high halogen demand, however, it is more economical for the demand to be satisfied with lower cost chlorine chemistry than with higher cost bromine chemistry. Thus, there is a need for a liquid composition containing both bromine and chlorine. This invention addresses this need as well.
There is also a need for a solid high-activity bromine-containing biocidal composition that is stable and fast dissolving. There are several solid, high-activity bromine-releasing compounds that are sold commercially as biocidal products. They are generally available as heterocyclic organic compounds to which an oxidizing bromine atom is covalently bonded to a nitrogen atom on the ring. Examples include N,N′ bromochloro-5,5-dimethylhydantoin, 1,3-dibromo-5,5-dimethylhydantoin, and mixtures of these compounds with various other components. In water, these materials hydrolyze to release hypobromous acid, which is the biocidal agent. However, the major limitation of these solid compounds is that they are only sparingly soluble in water. Indeed, N,N′ bromochloro-5,5-dimethylhydantoin has a water solubility of only 0.1% at 20° C. As a result, bromine is only released very slowly from these products as they dissolve. This is a significant disadvantage when the water requires treatment with a high, rapid dose of biocidal bromine as is necessary in shock and slug dosing procedures. Therefore, there exists the need for solid, high-activity forms of bromine of high water solubility and which dissolve rapidly and completely to provide the water with a high, rapid dose of biocidal bromine. Additionally, these solid products must have a long shelf life and not lose activity prior to use. This invention addresses these needs.